White discharge printing is a printing method achieving localized discoloration and getting diverse patterns through the way that dyed ground is printed with a paste containing discharging agents (normally reducing agent or oxidant), then the dyes are discharged destructively from the selected areas. Compared with direct printing, it receives more attentions due to its special printing effects that possessing dark ground and pastel shade, attractive small motif and fine line work, rich-layer patterns, boldly contrasting colors, higher sharpness and abundant ground shades.
The process of white discharge printing can be roughly divided into two steps. The fabrics are dyed, and then the designs are printed on the fabrics. Because of the existence of chemical agents (such as the strong reducing agent or oxidant, namely discharging agent), the ground shade is destroyed subsequently by steaming stage, and white patterns or dark ground and pastel shade is obtained, which makes up the defects of the direct printing.
Conventional white discharge printing is considered as a process with complicated procedures, following with numerous factors that affecting the effects of white discharge. That is to say conditions are difficult to control.
The main factors with effect of white discharge printing are the type of dye structure and its substantivity to fibres. Owing to the different stabilities or sensitivities of dye matrix structures to discharging agents, the capabilities of discharging of ground shades are directly influenced. Moreover, the size of decomposed products of dyes, the affinity to fibers, the color depth and so on, are directly affect the effect of white discharge printing. At the same time, desizing in the conventional white discharge printing also apparently affect the removal of decomposed product, as well as the effect of white discharge printing. In addition, the selection of discharging agents in printing pastes and types of fabrics, control of steaming process are also have an impact on the effect white discharge printing.
Conventional discharge printing not only makes large demands on consumption of chemicals, but also the energy, which account for higher costs. Meanwhile, varieties of residual chemicals in the printing pastes would cause a great threat to the environment.
Consequently, instead of the conventional process, providing an efficient and green environmental protecting method of white discharge printing process can decrease the energy consumption, reduce the applications of common chemicals and facilitate the achievement of cleaner production. Hence, it has an important significance of realization of energy saving and emissions reduction in enterprises of textile dyeing and printing.